1. Field of the Invention
The present invention relates to a quadrature phase modulator which generates quadrature phase modulating signals in digital type using a counter and an adder.
2. Description of the Prior Art
In general, with the advent of a satellite broadcasting and a mobile communication, it has become very important to transmit and receive data between a transmitter and a receiver. In order to transmit and receive data between a transmission terminal and a receiving terminal, data which are modulated in the transmission terminal are transmitted to the receiving terminal using wire or radio channels. Since this digital communication method is not nearly influenced by external noises, it is widely used to clearly reconstruct the transmitted signals on the receiving side. In methods for modulating the digital signals, there are ASK(Amplitude Shift Keying), PSK(Phase Shift Keying) and FSK(Frequency Shift Keying).
PSK, which is a technique that modulates the phase according to the phase shift corresponding to the digital data, is commonly used in radio communication, such as a satellite capable of transmitting signals in low power.
The quadrature phase modulation is commonly used in satellite broadcasting because it can transmit data by two bits in only a form of sine wave, and then decrease the bandwidth of the transmission channel.
FIG. 1 is a view illustrating the principle of a conventional quadrature phase modulator. Four types of sine wave forms are generated according to input digital data. In FIG. 1, wave form A, B, C and D, which are respectively shifted out of phase by 90 degrees, denote input digital data 11, 10, 01 and 00, respectively. That is to say, the quadrature phase modulation manner generates one sine wave form by 2 bits corresponding to the input digital data multiplied by the carrier frequency (sin Wct, cos Wct).
FIG. 2 is a view showing a conventional quadrature phase modulator which consists of a serial-to-parallel data converter, a delay device, a low pass filter, an oscillator, amplifiers, multipliers, an adder, a band pass filter, an oscillator, amplifiers, multipliers, an adder, a band pass filter and a phase shifter. In FIG. 2, the serial-to-parallel data converter converts serial input digital data into parallel data I.sub.-- OUT and Q.sub.-- OUT. Data I.sub.-- OUT and Q.sub.-- OUT, which pass through the low pass filter and the amplifier, are respectively converted into sine wave forms. Two signals having a phase difference of 90 degrees are generated by the oscillator, and each signal is multiplied by the output of the amplifier. Data I.sub.-- OUT and Q.sub.-- OUT passing through the amplifier are added up at the adder and pass the band pass filter and are transmitted to a channel through the amplifier.
However, the conventional quadrature phase modulator has problems in that the chip increases in size because of the use of the low pass filter, the band pass filter and the oscillator, and the adder decreases slows down it.